1. Field of the Invention
This invention relates to a method and apparatus for measuring polarization-mode dispersion in optical devices such as optical fibers or components.
2. Discussion of the Prior Art
Polarization-mode dipersion (PMD) is a fundamental property of optical fiber and components by which any lightwave signal is split into two polarization modes that travel at different speeds on the basis of polarization state. The two polarization modes experience a difference in propagation time known as differential group delay (DGD). The PMD of an optical fiber or component is simply the average DGD. Existing PMD, measuring techniques [see, for example Y. Namihira et al, Electron Lett., 1992, 28, No. 25, 2265-2266 (1992) and A. Galtarossa et al, J. of Lighwave Technol., 14, 42-42 (1996)] utilize the fixed-analyzer method, the Jones-matrix method, and the Poincare-sphere method. A major limitation of these methods is that any motion of the measuring apparatus, especially at the end of fibers, can totally destroy the measured results. Maintaining a motionless condition is often difficult, especially with field measurements. One motionless dispersion measuring technique based on measuring four-wave mixing (FWM) products in a low-dispersion, low PMD measurement fiber has been proposed [see S. Song et al, J. of Lightwave Technol., 17, 2530-2533 (1999)]. The polarization-mode dispersion measurement accuracy of this technique is limited by an additional PMD added by the measuring fiber itself, which is used to generate four-wave mixing signals. Moreover, the PMD measurement accuracy of the method is also limited by the wavelength tuning range of the variable signal because the four-wave conversion efficiency drops rapidly with wavelength detuning, causing a reduction in the intensity of the FWM signal and a degradation in the optical signal-to-noise ratio (OSNR).